Methods of water diversion employed in waterflooding depend in part on the degree of heterogeneity of the porous media or strata being treated. Relatively more permeable zones of the subterranean formation tend to take most of the injected fluids. Initially, this is acceptable in sweeping oil from such zones of relatively high permeability. However, this subsequently becomes undesirable as the oil content of such strata becomes depleted since much of the subsequently injected flood water or other fluid by-passes the relatively less permeable but oil-bearing zones and provides little benefit in enhancing further hydrocarbon recovery.
An isolated high-permeability zone or fracture can be plugged at the well core face by a shallow layer of applied cement, though such a permanent relatively irrevocable technique often is considered undesirable. More desirably, a communicating high-permeability zone preferably is plugged to some considerable outward lateral depth to be most effective in preventing floodwater or gas from otherwise merely flowing around a narrow shallow plug and back into the high-permeability or swept zone. In depth plugging of a relatively high-permeability zone converts the zone into a zone of much lower permeability. Subsequently injected floodwater or other fluid then will tend to enter formerly by-passed but now relatively more-permeable hydrocarbon-bearing zones and thus mobilize increased amounts of hydrocarbons therefrom.
In depth plugging can be effected by the injection of gelable thickened aqueous solutions containing sequestered polyvalent metal cations which cause the gelation or crosslinking of the thickened aqueous solutions when the pH of the solution is in a gelation pH range.
The injection of gelable systems triggered by a following aqueous acidic solution for in-situ pH adjustment has been used. However, this sequence may result in gelation occurring so rapidly and shallowly that a sufficient lateral outward depth of plugging is not effectively obtained in the most permeable strata where needed. On the other hand, when the acidic component is premixed with the gelable composition, gelation also can be too swift, resulting in the necessity of shearing the gelled polymer in order to be able to obtain adequate injection, but which reduces the effectiveness of the gel.
Needed is a method of in-situ gelling a gelable injectable aqueous liquid composition which has an initial pH outside the gelation pH range yet which possesses the capability of forming the desired gel in-situ, and without the need for premixing with acid or the follow-up injection of acid.